The protein-associated DNA single and double-strand breaks induced in mammalian cells by DNA intercalating agents and epipodophyllotoxins have been shown to represent an effect on topoisomerase II. The aim of this project is to investigate the relationship between the drug-induced protein-associated DNA strand breaks and the physiological state and pharmacological sensitivity of cells. Pharmacological concentrations of epipodophyllotoxins (VP-16 or VM-26) appear to produce only topoisomerase II-mediated DNA breaks in mammalian cells in culture. The association of topoisomerase II with nuclear DNA can be estimated by alkaline elution studies measuring m-AMSA- or VP-16-induced protein-associated DNA breaks. L1210 or DC3F cells seem to have higher levels of topoisomerase II than HT29 or VA-13 cells. Proliferating cells and S-phase cells seem also to have higher topoisomerase II levels than arrested or G0-G1 cells. A good correlation was found between the yields of topoisomerase II-mediated DNA breaks and the cell killing produced by a variety of antitumor drugs. Studies of resistant cells (DC3F/9-OHE) suggest that the sensitivity of cellular topoisomerase II to antitumor drugs could be regulated by other nuclear proteins, such as topoisomerases I.